1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a projection lens.
2. Description of Related Art
The design of a projection lens is challenging in many ways to an optical designer. In general, the projection lens in a projection device would be designed to have low distortion, high resolution, high contrast, and uniform image luminosity in order to achieve an optimal image quality. Moreover, besides a good image quality, a projection device would offer a large field of view (FOV) in order to project large images within a small space. However, a large FOV may result in large distortion. Furthermore, in order to increase the use efficiency of the light source and the uniformity of the image luminosity, the chief ray at the reduced side of the lens system would be designed as parallel to the optical axis as possible. Namely, the telecentric angle between the chief ray at the reduced side and the optical axis would be designed as small as possible.
However, aforementioned requirements in designing a projection device contradict each other. For example, in order to reduce the distortion of the projection lens, the number of lenses and the FOV of the projection lens are restricted, and as a result, the projection device may not meet the requirement of projecting large images within a small space. In order to increase the FOV and make the chief ray at the reduced side parallel to the optical axis in a projection device, the length of the projection lens and the number of lenses adopted would be increased so that the size of the projection lens may not be reduced. Moreover, conventionally, at least 9 glass lenses are adopted for fabricating a projection lens so as to achieve a low distortion, a high resolution, a high contrast, and a uniform image luminosity. However, such a design increases the cost and the volume of a projection lens.
The Taiwan publication patent No. 200925646 and No. 200839411 and the Japanese patent No. 8-313810 and No. 2003-131129 respectively disclose a projection device having three lens groups. Besides, the U.S. Pat. No. 7,009,777 and No. 5042929 respectively disclose a design related to lens groups.